The present invention relates in general to grouted pull-up mine roof or rock bolt anchor mechanisms of the post tensionable type, and more particularly to mine roof or rock bolt anchor mechanisms of the grouted or resin anchored type adapted to be introduced into a bore hole and having an anchor portion adapted to be anchored in the bore hole by a quick-setting grouting or adhesive composition and having a threaded bolt shank portion projecting externally from the bore hole about which a bearing plate and internally threaded nut may be assembled to draw the plate and nut upwardly to achieve desired compression force in the rock mass and which gives visual indication of the proper or improper relationship of the components.
In the past, two principal approaches have been used in providing anchor bolts for strengthening roofs in a coal mine, a subway tunnel or similar subterranean structures or to strengthen a rock mass. One type of construction employed parts that are expanded upon insertion within a drill or bore hole to provide a mechanical type of expansion gripping action, permitting a bolt or screw to be tightened upwardly against a face of the rock structure to achieve strengthening of the rock mass. The bolting assembly was required to enable a tension pull-up or tightening down of the bolt and bearing plate assembly in compliance with Bureau of Mines Standards established for the industry, including requirements that such an anchor bolt unit be capable of achieving installed bolt tensions within the range of at least 6,000 to 8,000 pounds.
A more recent type of assembly involves the use of adhesive or grouting material within the bore hole or drill hole for securing the rod or bolt in place, and if pull-up tensionsing was required, the outer end of the rod or bolt was threaded and provided with a nut and faceplate washer, permitting the plate to be pulled up against the front face of the rock mass or mine roof when the nut was tightened.
The mechanical type of anchor bolt described above has been somewhat limited in regard to the conditions under which it can be used, as it requires a relatively strong or more stable type of stratum and has not proven satisfactory for many types of formations encountered, particularly where mudstone or siltstone, limestone and laminated sandstone or shale formations occur. The second or adhesive type of anchor bolt unit has the advantage that the adhesive may be employed to strengthen the particular rock formation as well as secure the bolt in position in the bore hole, but such an adhesive type unit involving fast setting resin adhesives has been very expensive and has required great amounts of adhesive when applied in accordance with the customary practice of substantially filling the hole about the full length of the rod of bolt member.
There has been a general recognition of the need for improved types of bolting assemblies which will make possible the achievement of greater amounts of face compression and draw up tension strength, and which will also reduce expense from the standpoint of installation time and labor and materials.
In an effort to provide a bolting unit of the resin type which would achieve improved compression strength and draw up tension strength, and seek to reduce expense of material and labor and time involved in installation, a type of resin anchored bolt assembly has been proposed in earlier U.S. Pat. No. 3,877,235 wherein a threaded anchor unit was provided with a hollow cylindrical leading portion resembling a pipe-like shell, which was assembled to an anchor bolt and provided with a restraining device that permitted the threaded leading end cylindrical anchor portion to rotate with the anchor bolt, without relative rotation between them, whereby the shell-like anchor portion may be inserted into the bore hole behind a capsule or cartride of adhesive resin and employed to crush and mix the resin, whereby, when the resin has cured to adhesively secure the anchor portion in the bore hole, application of a predetermined amount of torque to the bolt cause the restraining device to release the bolt for rotation relative to the anchor device and tighten it to generate compressive force in the rock mass. This was achieved by providing a fracturable disc member forming a bottom wall of the pipe-like hollow cylindrical anchor portion to which a standard nut was welded, so that when the anchor portion was held by the resin against further rotation in the bore hole, sufficient torque could be applied to the bolt to fracture and penetrate the frangible disc and allow the bolt upon tightening to be drawn up into the bore hole to produce the desired compressive force effects.
Later U.S. Pat. No. 4,051,683 discloses another type of resin anchored bolt assembly wherein a threaded anchor unit including a reinforcing rod and a threaded cylindrical collar or coupling member is provided with a transversely extending shear pin forming a stop restraining advancement of the bolt into the collar or coupling member until the resin cartridge is broken and the resin adhesive sets sufficiently to securely fix the reinforcing rod and collar or coupling member against rotation in the bore hole, whereupon advancement of the bolt shears the pin so that a threaded end portion of the bolt extending into the collar or coupling member can thread upwardly into the collar or coupling member for a limited distance. Both of these prior resin type anchors require special construction of the anchor portion to provide the frangible disc or to provide the coupling and shear pin structure, entailing added manufacturing costs.
U.S. Pat. No. 4,023,373 shows another resin anchored bolt assembly using a stop nut with thin wall fingers engaging the bolt threads to restrain the anchor member against relative rotation on the bolt until the resin sets, whereupon further rotation of the bolt with sufficient torque will cause the stop nut to "shear or strip over thread stop 20 and slide down the smooth bolt stem portion 16 thereby releasing the stop means".
A further U.S. Pat. No. 3,979,918 discloses a grouted mine roof or rock bolt anchoring assembly which may be tensioned after installation of the anchor portion within the bore hole and setting of the grouting or resin adhesive, simply by threading an internally threaded nut member onto externally projecting external thread portions of a rod extending from and forming part of the anchor assembly, but wherein the externally threaded end portion of the rod simply extends through and projects beyond the lower end of the nut as the nut is threaded tightly up against the rock face surrounding the entrance to the bore hole.
An object of the present invention is the provision of an improved mine roof or rock bolt grouted anchor assembly of the post tensionable type, wherein an externally threaded rod member extending from the anchor portion to be anchored by the grouting or resin material in the bore hole projects to an exposed position beyond the bore hole entrance, and bearing plate means and an internally threaded nut is assembled onto the threaded end of the anchor rod portion and includes a frangible disc incorporated therein which is forced out during tightening of the nut to proper tensioning position on the anchor rod portion.
Other objects, advantages and capabilities of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings illustrating a preferred embodiment of the invention.